There is an important difference between sodium concentration and sodium activity. The concentration simply refers to the total amount of sodium present per unit volume, whereas the activity refers to the amount of free sodium ions that are free to enter a chemical reaction and are not complexed to other plasma constituents, e.g. proteins. Due to historical convention sodium tends to be reported in terms of it’s concentration rather than true activity.
Believe it or not, if we were able to measure the exact amount of sodium in a litre of blood, it would only be in the region of 84mmol. This is because blood contains around 40% red cells, which have little or no sodium content.
If we then tried to figure out the concentration of sodium in the remaining 0.6 litres of ‘fluid’ in blood (84/0.6) we would get a figure of 140mmol/L.
However, remember that in normal circumstances plasma is only 93% fluid; the remainder being composed of proteins, lipids etc.. So, in actual terms, the true plasma water sodium concentration is (140/0.93), or around 151mmol/L.
This knowledge will help explain the concept of pseudohyponatraemia associated with hyperlipidaemia or hyperproteinaemia. In these scenarios, the plasma water content per litre of plasma is reduced from 93% to let’s say 80% for example.
If the lab reported that our patient’s sodium was 120mmol/L then we can try to calculate what the true plasma water sodium concentration is:
120/0.8 = 150
This is similar to the true plasma water figure in our normal patient, who had a sodium of 140mmol/L and plasma water content of 93%.
This should also explain why we call ‘normal saline (0.9%)’ an isotonic fluid, even though when you read the chemical composition label, you will find that it contains 154mmol/L of sodium ions and 154mmol/L of chloride ions.
Next week I’ll try to delve into the various ways that sodium has been measured in clinical laboratories.